The present invention is in the electronic single electron switching field in which device operation is based on one-by-one transport of electrons through a small sub-structure. According to the invention, high temperature (higher than room temperature) single electron electronic switches have been operated. No such capability has been previously demonstrated, and is produced in the invention through a new class of material, referred to herein as silicon nanoparticles.
The switch is one of the most important components of any electrical system. Electrically, the switch is a two-state device. It is either ON or OFF. An ideal switch offers no resistance to current when it is in the on state, but it offers infinite resistance to current when it is in the off state. These two states represent the limiting values of a resistor. The ON state corresponds to a resistor with a numerical value of zero, and the OFF state corresponds to a resistor with a numerical value of infinity. Transistors presently used as switching devices provide some resistance to current flow in the ON state.
A quantum well can act as a very sensitive switch. FIG. 1a represents a quantum well device by a well connected to two sources of electrons A and B via a barrier on each side. The frames in FIGS. 1a-1b and FIG. 2 describe the I-V characteristics for the quantum well device. With zero voltage between points A and B (FIG. 1a), the quantum well presents a barrier to the n-carriers. There is no tunneling because the energy levels on both sides of the quantum well are occupied equally. When a voltage is applied, the n-carriers on the left are at a higher energy (FIGS. 1b-d), and they can tunnel into empty levels. Thus, the current rises with V, as shown in FIG. 2. When the bottom of the conduction band on the left is lifted high enough to match one of the low lying quantized levels inside the well, there is a drastic increase in the flow of current. A further voltage rise decreases the current, because the bottom of the conduction band on the left no longer matches a well level. As the voltage is increases further, the current decreases and then increases again as the second energy level is approached. The effect is very similar to the resonant behavior that occurs when the frequency of a harmonic driving force matches the characteristic frequency of a slightly damped harmonic oscillator. The resonance makes for a sensitive switch.
Electronic devices and circuits are getting smaller and smaller. Such devices are required to work faster and consume and dissipate less energy. The goal is to attain single electron devices to reduce the impedance of a switch in the on state, to decrease response time, and to decrease power consumption. In single electronics (single electron technology), device operation is based on the concept of one carrier for one bit of information. In August, 1991 Eigler announced the simplest and probably the smallest switch that it is possible to make (Nature, vol. 352, p 600). The on and off states of his atomic switch rely on the position of a single Xenon atom. The atom can be switched between two stable positions representing ON and OFF. One position is in a kink on the surface of a crystal of nickel, the other is on the tip of a STM (scanning tunneling microscope) held still just a few atomic diameters above the surface. By applying a short pulsed voltage to the tip, Eigler could make the Xenon atom jump across the gap from the surface to the tip. A pulse in the other direction made the atom jump back down onto the surface. While the atom switch is a good demonstration of what is possible with an STM, it is unlikely ever to form a useful device. The Xenon atom is held onto the surface only by very weak chemical bonds. This means that the whole process has to take place at temperatures as low as 4xc2x0 Kelvin so that thermal vibrations do not bounce the Xenon around, and in a vacuum so that gas molecules do not disturb it. This makes the whole apparatus very bulky. Modern semiconductor chips cram millions of switches into an area less than a centimeter square and must be durable enough to survive in a electronic devices used in everyday electronic application environments. Obviously, Eigler""s STM single atom switch is unsuitable for such applications.
An alternative to a single atom switch is one that relies on a single electron. American researchers made such a transistor several years ago, employing techniques used to make advanced semiconductor chips. Robert Jaklevic, however, has built a single electron device incorporating an STM tip similar to the way Eigler used a tip as one of the electrodes of his atomic switch. Jaklevic, of the Ford Motor Company""s Scientific Laboratory in Dearborn, Mich., was working with colleagues from the University of Michigan at Ann Arbor. Jaklevic""s team took a semiconductor substrate coated with a thin layer of an insulating material and sprayed minute blobs of indium onto it. It then used the STM tip to locate one of the tiny blobs and positioned the tip over it. By manipulating the voltages applied to the tip and the substrate, the researchers can control the movement of single electrons in and out of the blob.
In the last two years, it has been expressed that the physics of a quantum dot (particle, or well) connected to leads is similar to the physics of a magnetic impurity in a metal host. In the process, an electron in the impurity (or the quantum dot) couples to an electron in the metal host (or electrode) to form a bound singlet magnetic electron state. The Anderson treatment of the problem simply models it, in essence, as a discrete-continuum (band) coupling, i.e., the discrete energy levels of an impurity or those of a quantum dot couple to a continuum of states of a conductor. One dramatic manifestation of the process is that the electronic transport through the impurity or dot exhibits, below a certain temperature (Knodo temperature Tk), strong conductance enhancement and even perfect transparency (switching action) near zero bias if it is symmetrically coupled to the conductor or leads. The temperature Tk is defined as Tk =(U*)xc2xdexe2x88x92*(*xe2x88x92*)/2 * where U=e2/C is the single electron charging and C is the capacitance of the particle, * =*1 =*2 is the coupling of the dot to the leads as a tunneling rate of electrons, * is the electronic energy in the quantum dot, and * is the chemical potential energy of the leads at equilibrium.
Due to the magnetic nature of the process, the conductance is maintained in the presence of a magnetic field but gets split into two energy conductance channels, slightly shifted above and below zero biasing. Another system which was recently shown to exhibit the Kondo effect is electron charge trap state. The charge traps are created in a dielectric such as Si3N4 by field breakdown, while the dielectric is sandwiched in the metal by advance nanolithography.
Although the coupling between a magnetic impurity and a conduction-electron sea, as in the Kondo effect in a magnetic impurity in a host metal, has been the focus of condensed matter research for many years, only in the last two years experiments have been conducted on the analogous problem of a quantum dot connected to two leads. The binding energy of the magnetic singlet state is expected to be small for quantum dots since the electrons in the lead and the quantum dot are separated by a large distance, and thermal agitation will suppress the process at any temperature. Only recently have advances in fabrication made it possible to fabricate small enough quantum structures that permit the singlet state to be stabilized at finite temperatures.
For example, the Kondo process has been observed in the electronic transport between the source and the drain of a sub micron single-electron transistor (SET) that is constructed from GaAs/AlGaAs material by Van der Vaat et al., Physical Review Letters 74,4702. (1995). The quantum dot in the device is a sheet of electrons (two dimensional electron gas) of approximately 150 nmxc3x97150 nm, containing xcx9c40 electrons. Those are confined at the GaAs/AlGaAs heterostructure interface. With this structure all of the Kondo-like physics have been observed including the electronic transparency and the magnetic properties, but only at temperatures below 1 degree Kelvin.
More recently, advances in down scaling of Si based transistors opened the way for similar studies to be conducted on silicon based FET devices. The electron gas in the silicon case is confined at the Sixe2x80x94SiO2 interface by applying a voltage to the gate electrode. The enhancement in the differential conductance at zero bias was observed. However, it was not clear if this was due to a Kondo effect. Unlike the GaAs/AlGaAs system, the conductance did not show the characteristic magnetic features. An imposed magnetic field tends, in general, to destroy the conductance instead of just splitting it into two energy channels.
Quantum dots made of an electron gas confined at the interface of heterostructures as in transistor systems have certainly provided a means to observe the Kondo effect. However, characteristic sizes of such structures are limited to 100 nm, limiting the temperature below which the singlet magnetic state may be bound to sub one degree Kelvin (several hundred milli Kelvin). Moreover, the electron gas involves a few dozen electrons, with confinement in one dimension only. As a result, switching via the Kondo effect has remained in the realm of low temperature physics.
To bring the switching effect to the realm of room temperature, real topographical ultrasmall quantum particles or dots with large U, (small C), and strong *xcx9c(*xe2x88x92*) must be used. Such particles are provided in the present invention. The particles are the basis for a switch that provides a single electron transparency and fast switching operation at room temperature.
The present invention relies upon a previously unknown material, uniform silicon nanoparticles dimensioned at 1 nm (about 1 part per thousand being of greater dimension). This new material and a method for making the same are described in copending application Ser. No. 09/426,389, to Nayfeh et al. entitled SILICON NANOPARTICLES AND METHOD OF MAKING THE SAME. That application is incorporated by reference herein. The silicon nanoparticles have a discrete set of states resulting from the quantum mechanical wave-like nature of electrons, capable of capturing/emission single charge carriers. For a given material, the energy amount is the basic unit E=3h2/8 md2, where m is the mass of an electron, h is Planck""s constant, and d is the diameter of the particle. For the particles of the invention, dxcx9c1 nm, providing an energy spacing of 1 electron volt.
This energy spacing extends single electron switch properties beyond the low temperature process demonstrated in the prior art. We have demonstrated an electronic fast switch for operation at room temperature utilizing the silicon nanoparticles sandwiched between two conducting electrodes. The silicon nanoparticles, when on an n-type silicon substrate exhibit, at zero bias, a large differential conductance, approaching near full transparency. The conductance is observed after one of the electrode is first biased at a voltage larger than 3.1 eV (switching voltage), otherwise the device does not conduct (closed).